1. Field of the Invention
This invention relates to a DC/DC converter which converts an input power supply voltage into a prescribed DC voltage by the turning on and off of a switching device.
2. Description of the Related Art
As one design for a DC/DC converter, a switching device is provided between a terminal to which a power supply voltage is input and a terminal to which a prescribed DC voltage is to be output, and by turning on and off (closing and opening) this switching device, the prescribed DC voltage is maintained. This design affords compactness and high efficiency, and so has come into widespread use (for example, Japanese Patent Laid-open No. 7-336999 and Japanese Patent Laid-open No. 11-332222).
A conventional DC/DC converter of this design is shown in FIG. 4. In the DC/DC converter 101, the switching device 114, which is a PMOS transistor, supplies power to the load side from a power supply (VCC), and under prescribed control, that is, by closing and opening the switching device according to the output from a comparator 110 described below, maintains the load-side output (VOUT) at an output setting voltage. A smoothing circuit 115 comprising a coil, capacitor and diode is connected to the switching device 114, to smooth the voltage from the switching device 114. The load-side output (VOUT) is voltage-divided by a voltage divider 116 comprising series resistances, and is input to the inversion input terminal of an error amplifier 111. The error amplifier 111 amplifies and outputs the error between the voltage of an error comparison reference power supply 118 and the voltage input to the inversion input terminal. The output voltage (VFB) of the error amplifier 111 is input to the comparator 110. In the comparator 110, the output voltage (VFB) of the error amplifier 111 and a triangular waveform voltage (VS) are compared, and the output voltage (VC) passes through a buffer 119 and is input to the gate of the above-mentioned switching device 114. In this way, the switching device 114 is controlled by a feedback circuit from the load-side output (VOUT).
In the comparator 110, the triangular waveform voltage (VS) compared with the output voltage (VFB) of the error amplifier 111 is generated and output by a triangular waveform generator circuit (TRI).
If the output voltage (VFB) of the error amplifier 111 is higher than the triangular waveform voltage (VS), then a low level voltage is output from the comparator 110 and passes through the buffer 119, and a low level voltage is input to the gate of the switching device 114. As a result, the switching device 114 is closed (turned on).
Conversely, if the output voltage (VFB) of the error amplifier 111 is lower than the triangular waveform voltage (VS), then a high level voltage is output from the comparator 110 and passes through the buffer 119, and a high level voltage is input to the gate of the switching device 114. As a result, the switching device 114 is opened (turned off).